The combination of a bidirectional converter with storage capacitor, acting as an active filter, with a conventional multiphase converter, acting as a power source, is described in Ayyanar et. al. U.S. Pat. No. 8,243,410 and Lambert et. al. Lambert et. al. describe control of the bidirectional converter (active filter) as being primarily that of a voltage-controlled current source or sink, with the voltage control based on the output voltage. The active filter then becomes a shunt capacitor as seen from the main converter (power source). The design of the main converter control system is then performed in a conventional fashion, accounting for the additional effective capacitance of the active filter in the specification of the compensation network for the power source. A second loop is provided within the control of the active filter to regulate the voltage on the auxiliary (storage) capacitor. This second loop is apparently slow compared to the main current control, as suggested by e.g. FIG. 3 of Lambert et. al. This second loop is also optionally adjusted to support load-line control (voltage positioning) of the auxiliary capacitor, using the primary converter (power source) current as an input to adapt the target voltage for the auxiliary capacitor (Lambert et. al. equation (34)).
This control approach simplifies the design of the overall controller, since the control system for the active filter is not dependent on the state of the power source, and the power source only sees the active filter as a change in load. Thus the two converters may be controlled by independent compensators, using conventional single-input-single-output (SISO) design techniques. However, it is well-known that dynamic voltage scaling (DVS), in which the supply voltage to a digital circuit or portion thereof is changed depending on the instantaneous requirements of the digital circuit, is useful for optimizing performance and power consumption of digital systems. The control approach described by Lambert et. al. provides good response to load current transients, but an intentional change in output voltage must be managed by the power source converter. As noted by Lambert et. al., the primary (power source) converter is designed for efficiency at DC and low frequencies, and thus uses slow switching frequency and large inductances. It is not optimized for rapid changes in output voltage. Further, if the output voltage is to be changed, the low-voltage transient processor (active filter) must be made inactive, or its control configuration must be changed, to prevent it from drawing large currents and over-charging or depleting the storage capacitor, and slowing the change in output voltage driven by the primary converter.
There is a need for a more robust control approach for the coupled power source/active filter that allows fast DVS, while ensuring that the active filter operating constraints are satisfied, and providing good stability and response to load current transients, including rapidly repeated load current transients.